How to Grow Dahlias

Dahlias are late-season bloomers. They bloom from mid-summer through the first frost and are available in a vast array of colors, patterns, bloom sizes, and flower forms. Plant size ranges from compact border dahlias to plate-sized blossoms atop 6-foot plants. Native to Mexico and Central America, over 20,000 dahlia cultivars have become darlings of plant breeders and flower shows alike.

Most of us think of dahlias as cut flowers, and while they do make for exceptional arrangements, they also deserve recognition as garden plants. Although they can be temperamental and fussy about the weather and their growing conditions, many newer cultivars have been bred to be more reliable and easy-growing.

While often labeled as perennials, dahlias are actually too tender to leave in the ground all winter in USDA zones 7 and lower, where ground temperatures dip below 20 degrees Fahrenheit. If you live in a colder climate, you either have to dig and store the tubers indoors over the winter or treat them as annuals and replace them each year in the spring. Tubers are planted outdoors in spring after all danger of frost has passed. The plants reach maturity and bloom in about eight to ten weeks after planting.

Dahlia Planting and Care

For such stunning flowers, dahlias are actually fairly easy to grow. Dahlias grow from tubers, not bulbs, which you can start them indoors in early spring and move or plant them outdoors once the danger of frost has passed. (Likewise, you can wait until the soil has warmed and dried a bit in the spring, then plant the tubers directly outdoors.)

Plant large tubers 6 to 7 inches deep, while the smaller tubers can be planted a bit more shallow: 2 to 3 inches deep. Lay single tubers horizontally in the planting hole with the eye pointing upwards, and plant tuber clumps upright and vertically with at least one inch of soil covering last year's stem. You can also grow compact dahlia varieties in containers.

Space dahlias tubers 3 to 4 feet apart for the larger varieties and 2 to 3 feet between rows. Dahlias require good air circulation to avoid powdery mildew and other air-borne diseases. If you're growing dahlias in containers, the bigger the container the better. A rule of thumb is for the container to be 12 inches deep and wide for each tuber.

Regardless of where you grow dahlias, in the ground or in containers, ensure they thrive by planting the tubers in an area that receives full sun and by watering deeply and feeding regularly when new green growth appears. Within a few months, you'll be treated to the best reward—a garden full of stunning, colorful, and oversized blooms.

Light

In order to produce an abundance of blooms, dahlias require full sun, preferably at least 6 to 8 hours a day. In hotter climates (primarily USDA zones 8 and up) they will benefit from a bit of shade during the peak afternoon hours when the sun is especially hot.

Soil

Dahlias prefer rich, loamy, well-draining soil with plenty of organic matter and a fairly neutral soil pH of around 6.5.

If you are planting dahlia tubers ahead of time in containers to get a jump start on the growing season, mix in a bit of peat moss or vermiculite into the potting soil for better moisture retention—potting soil can dry out quickly and it's especially important that tubers have a bit of moisture while they developing a root system and begin to sprout.

Water

Do not water dahlia tubers until they begin to sprout green growth above the surface of the soil. Once you plant tubers in the spring, let nature take it's course until sprouting occurs. The tubers don't need water until they sprout and develop a root system.

Once sprouted, dahlias require watering once or twice per week. Water deeply because some large tubers are planted six inches deep and water needs to reach the root system. If summer days are especially hot and dry, you might need to water more frequently. It's important to never let the soil dry out because a dry top layer of soil equals a dry plant.

Temperature and Humidity

Timing is especially important when it comes to planting dahlias, as they'll struggle to establish in cold soil. Wait until the final spring frost has passed and the ground temperatures have reached 60 degrees Fahrenheit. You can start tubers indoors, perhaps in a garage or greenhouse, in containers to get them off to a quick start. It's safe to plant or place them outdoors when all danger of frost is past.

If you plan to dig and divide tubers at the end of the season and store them indoors over winter, they require some humidity so they don't shrivel and dry out. During the growing season, however, they don't require any extra humidity in order to thrive outdoors.

Fertilizer

Dahlias are heavy feeders—the more food they receive the larger the plant will grow, and subsequently the bigger (and more numerous) their flowers will be. Whatever type of fertilizer you choose, do not use one with a high percentage of nitrogen. Too much nitrogen creates lush foliage but few blooms. Use a fertilizer with a higher percentage of phosphorus (perhaps a 10-30-20 ratio) to promote blooming.

If you plan to dig and store tubers over winter, stop fertilizing your plants at the end of August. You don't want to encourage more growth too late in the season and you want to prepare the tubers to go dormant after the first frost kills the foliage.

Harvesting Blooms

Dahlia blooms are ready to be harvested once the flowers are 3/4 to fully opened: they don't open much more after they are cut. Cut deeply down into the plant for the longest stems; don't be afraid to sacrifice the small side shoots. The longer the stems you cut, the more long stems will grow back. Harvesting blooms encourages more blooms and more branching.

To ensure the longest-lasting floral arrangements, cut blooms in the morning when temperatures are cool and the plants are full of water. When you bring the cut stems indoors, plunge them into 2 or 3 inches of very hot tap water (not boiling) to seal them. Leave them there until the water cools, then arrange them in a vase or other floral arrangement container.

If you don't plan to cut blooms to create floral arrangements, be sure to deadhead faded blooms to keep the plant looking tidy and to encourage more blooms.

Common Pests and Plant Diseases

The pests that affect dahlias the most are typical to other similar seasonal blooms. A common pest can be slugs, especially while the dahlia foliage is young and tender. Once the plants mature, slugs are usually not problematic. Earwigs, caterpillars, and thrips can also pose a problem. Some gardeners claim that deer love to eat their dahlias and others claim the deer avoid them. The deer palate might depend on what else is available to munch on in your garden, but keep your plants protected just in case.

Dahlias can also be prone to powdery mildew and other fungal diseases. Keep the foliage as dry as possible and space your plants to provide for good air circulation. If you notice an infection, treat it with neem oil or another natural solution.